1. Field of the Invention
The present invention relates to an apparatus and method for estimating a phase error; and, more particularly, to an apparatus and method for estimating a phase error based on a variable step size, which provides improved convergence speed and tracking speed even in mobile channel environment by variably applying the step size for phase error estimation according to channel status.
2. Description of Related Art
Since a turbo code is superior in view of performance and flexibility, it has been adopted as standards of various wireless communication services. In particular, carrier synchronization should be perfectly achieved in a synchronization block of a demodulator in order to obtain ideal performance in a low signal-to-noise ratio (SNR).
In general, a phase error is estimated and recovered using data symbol for carrier phase synchronization in burst transmission such as Digital Video Broadcasting-Return Channel System via Satellite (DVB-RCS) standard. However, this method is difficult to operate in an environment where SNR is low and a residual frequency error is great.
However, if an iterative decoding scheme is used in a turbo decoding, it is possible to easily obtain probability information of a received signal which is helpful to correct a phase. As a representative example, there is an external single estimator using an extrinsic soft output information outputted in a turbo decoding in order for phase estimation. The external single estimator uses a Least Mean Square (LMS) algorithm having a fixed step size.
It is preferable that a channel estimation method is optimal to both estimation accuracy and convergence speed. However, the estimation accuracy and the convergence speed are in a tradeoff relationship. Specifically, in case of the LMS algorithm having the fixed step size, if the step size is increased according to high-speed fading environment or frequency error size, the tracking speed is increased but the accuracy is decreased. The “tracking” means following the phase variation when the phase is not at a constant value but is continuously changed.
However, in a mobile channel environment and a Doppler-shift environment based on time, the receiver does not have information on fast varying channel and terminal. Thus, the LMS algorithm having the fixed step size is not optimized.